Cathodic electrodeposition coating agents are used mainly in the preparation of anti-corrosive primers on electrically conductive substrates. In particular, when deposited cathodic electrodeposition coating layers are baked, a reduction in edge coverage may occur and, as a consequence thereof, there is reduced edge corrosion protection. To reduce this problem of edge corrosion protection, cathodic electrodeposition coating agents often contain additives that improve edge coverage or corrosion protection of the edges.
Cathodic electrodeposition coating agents also usually contain anti-crater agents added as a preventive measure or after contamination has occurred, as an additive to prevent formation of craters and similar surface defects caused by impurities in the cathodic electrodeposition coating layers.
Cathodic electrodeposition coating agents containing special mixtures of anti-crater agents are known from U.S. Pat. No. 5,098,537 and U.S. Pat. No. 5,415,750. The mixtures of anti-crater agents may be of such a composition that they result in a content of 0.1 wt-% to 2 wt-%, based on the resin solids of the cathodic electrodeposition coating agent, of one or more polyesters having beta-hydroxyalkyl ester groups, an acid value of 0 to 3 mg KOH/g and a weight-average molecular mass of 1000 to 10,000. When the corresponding polyester from Example 5 of U.S. Pat. No. 5,415,750 or from Example 6 of U.S. Pat. No. 5,098,537 is calculated, a content of 40 wt-% of lateral and/or terminal, branched aliphatic C9-hydrocarbon radicals in the polyester molecule is obtained in each case.
The invention is based on the desire arising from practical considerations to keep the number of additives to be added to the cathodic electrodeposition coating agents as small as possible. Surprisingly, it was ascertained that when special acidic polymers are added to cathodic electrodeposition coating agents, both an anti-crater effect and an edge coverage and hence edge corrosion protection improvement is obtained.